1. Field of the Invention
The present invention relates generally to improvements in the art of making bituminous substrates, such as roadways, employing hot mix asphalt in which the surfaces of the aggregate to which the bitumen is applied are treated in order to improve the binding characteristics between the aggregate and the bitumen binder and to render the bond more highly resistant to water.
2. Description of the Prior Art
Since the early 1930's, asphalt surface course has been widely used for both private and public roadways. Hot mix asphalt is comprised of sand as one component, an aggregate or mineral component, and asphalt. The type of aggregate used may vary widely depending upon such factors as the cost and availability of the aggregate, as well as the particular application of the asphalt. Granite, quartz and limestone minerals, for example, can all be used as an aggregate material in forming hot mix asphalt. Asphalt forms the continuous phase and acts as a binding agent for the mineral aggregate. The continued preservation of bond between the asphalt and rock or mineral content of the composition is necessary to insure a lasting pavement. Water, natural weathering and traffic loading are often responsible for adhesion failure which occurs between the asphalt and the aggregate.
Hydrated lime has been known to be an effective additive for reducing adhesion problems of the type previously described. In the period from about 1910 to 1930, there were common applications of 1-2% hydrated lime in asphalt paving mixtures. A few states specified hydrated lime in their state standards. Lime provided a combination antistripping agent and mineral dust filler. Unaccountably, the use of lime for this purpose nearly disappeared in the period from about 1930 to about 1957. Apparently, since mineral dust fillers were specified by state highway departments, asphalt contractors chose to use less costly fillers than hydrated lime. Certain organic antistripping compounds also replaced hydrated lime, in spite of their higher cost. See, Chemistry and Technology of Lime and Limestone, Boynton, 2nd Edition 1980, Chapter 11.
In the late 1950's, the Colorado Highway Department adopted the "immersion-compression" test for analyzing moisture effects on bituminous mixtures. This test measured compression of asphalt specimens both dry after oven curing and wet after immersion in water and demonstrated impressive advantages for hydrated lime with certain types of asphalt-aggregate mixtures. Thus, in more recent years, many states have specified the use of 1% hydrated lime in many of the public paving projects.
While various techniques have been employed to add lime to the aggregate component of bituminous mixes, by far the preferred method in actual use was to mix dry hydrated lime with the aggregate. Typically, about 1% by weight, based on the mineral component, was used. There are many problems associated with the use of dry hydrate including dusting and poor performance when mixed with dry aggregate. As a result, the State of California recently approved the use of hydrated lime slurry rather than dry hydrate. The dry hydrated lime is slurried with water at about 20-25% solids and the slurry is used to treat the mineral component of the hot mix asphalt.
While the use of hydrated lime slurry can help to eliminate dusting and other problems associated with the use of dry hydrate, other problems have remained. The extra water provided by a hydrated lime slurry is undesirable since the aggregate must by dried before mixing with the asphalt. Hydrated lime is more expensive than quicklime and can require the presence of extra tanks, silos and mixing equipment at the hot mix plant. Also, when quicklime is slaked to produce hydrated lime by a traditional slaking process, the resulting % solids of the slurry is usually low, on the order of 15-30% solids. This means that 5.7 to 2.3 times more water would be added to the aggregate being treated than lime. Thus, if 1% Ca(OH) 2 were being added to the mineral component at 15% solids, this would mean an increase in the moisture content by 5.7% which would require additional drying at considerable cost in money and production time. At 30% solids, addition of 1% Ca(OH) 2 would similarly mean the addition of 2.3% water.
The present invention has as its principal object to eliminate the previously mentioned shortcomings with lime slurry treatment of aggregate used in bituminous mixes, such as hot mix asphalt.
Thus, it is one object of the invention to provide an improved method for treating the aggregate used in hot mix asphalt through the use of a hot slurry prepared from quicklime rather than from dry hydrate.
Another object of the invention is to provide a slurry produced from quicklime for treating the mineral component of hot mix asphalt which slurry has a higher percent solids than traditional slurries used for this purpose.
Another object of the invention is to produce such an improved slurry from quicklime which has an increased temperature over hydrated lime slurries which can be added hot to the aggregate to cause increased evaporation of water and thus lower the requirements for additional heat to dry the treated aggregate.
Another object of the invention is to provide such a hot slurry having a lower viscosity, thus providing improved mixing of the Ca(OH)2 particles in the slurry with the sand and aggregate in the bituminous mix.